A wellbore system 20 shown in FIG. 1 has a casing string 22 cemented in a wellbore 10. The casing string 22 has a shoe 30 and a toe sleeve 40 at its end and has various sliding sleeves 50 disposed along its length. The toe sleeve 40 and the sliding sleeves 50 deployed on the casing string 12 can be used to divert treatment fluid to isolated zones of the surrounding formation.
To prepare the system 20, the casing string 22 is run into position in the wellbore 10, and cement is pumped down the casing string 22 ahead of a plug (P). The cement exits the shoe 30 and fills the annulus 12 between the casing string 22 and the wellbore 10. As it is pumped downhole to the shoe 30, the plug (P) does not open the various sleeves 40 and 50 before it eventually reaches the shoe 30. After the cement is set, the toe sleeve 40 and sliding sleeves 50 can be opened so fluid pressure pumped down the casing string 22 can create fractures 14 in the cement 12 and the formation at the ports of the sleeves 40 and 50.
The toe sleeve 40 is opened before the sliding sleeves 50 and typically opens using differential pressure. As shown, the toe sleeve 40 is normally placed at the bottom or “toe” of the casing string 22 with the shoe 30 at the end of the completion, which allows the assembly 100 to be “washed” into position during run-in. When pressure is applied to the casing string 22 once cemented in the wellbore 10, the toe sleeve 40 opens so fracturing operations can begin.
For their part, the sliding sleeves 50 can be opened using a number of techniques. For example, the sliding sleeves 50 can be opened using a shifting tool manipulated downhole on coiled tubing. Alternatively, operators can deploy setting balls to actuate the sliding sleeves 50 in successive stages up the wellbore 10. In this operation, each of the sliding sleeves 50 has a seat (not shown). When operators drop a specifically sized ball down the tubing string 12, the ball engages the sleeve's seat.
Fluid is pumped down the tubing string 22 by a pump system 26 of surface equipment at a rig 24. The applied pressure against the seated ball opens the sliding sleeve 50 so fluid can communicate out ports to the surrounding wellbore 10. Because the zones are treated in stages, the lowermost sliding sleeve 50 has a ball seat for the smallest sized ball size, and successively higher sleeves 50 have larger seats for larger balls. In this way, a specific sized dropped ball will pass though the seats of upper sleeves 50 and will only locate and seal at a desired seat in the casing string 22.
As noted above, the toe sleeve 40 is typically a differential opening sleeve. FIGS. 2A-2B illustrate an example of a toe sleeve 40 according to the prior art in closed and opened conditions. The toe sleeve 40 includes a housing 42 with an internal bore 44. A sleeve 46 disposed in the housing's bore 44 is held sealed in a closed position (FIG. 2A) relative to ports 48 by shear pins 47 or the like. When pressure is increased in the bore 44 relative to the external pressure to a predetermined level, the shear pins 47 break, and the sleeve 46 slides open relative to the ports 48 (FIG. 2B) by decreasing the volume of a sealed chamber 45. Despite the effectiveness of such a toe sleeve 40 of FIGS. 2A-2B for a wellbore system (20) as in FIG. 1, it is possible for the toe sleeve 40 to have difficulty opening when the sleeve 40 has been exposed to cement allowed to cure when the system (20) is cemented in the wellbore (10) in a manner as described above.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.